of rochester



Patented Nov. 2 1926.

" UNITED STATES.

wearer PATENT Fries.

JOHN I. CRABTREE AND GLENN E. MATTHEWS, OF ROCHESTER, NEW YORK, AS-SIGNOBS TO EASTMAN KODAK COMPANY, OF ROCHESTER, NEWYORK, A CORPORA-rroiv on NEW YORK.

No Drawing. Application filed May 6,

Our invention relates to photographic processes and more particularly toa method of indicating, under the light conditions necessary in aphotographic dark room, when the acidity of baths whose usefulness is atleast partly dependentupon their acidity approaches a limiting value.

We have found that the change, which is not ordinarily easilydetectible, can be noted promptly if there is added to the bath anindicator which is selected, not only by the degree of acidity at whichit changes color, but also by the differentiation between its colorsunder the light of definitely limited Wave length by which it must beinspected It is not .suflicient that the apparent colors are differentwhen viewed by daylight since the same apparent color sensation may beobtained by different combinations of light of different wave lengths;and it can easily happen that two colors appearing quite different bydaylight may look the same under the light of selected wave lengthswhlch happen to be common to both. It is therefore necessary to choosean indicator which has markedly different transmissive properties forlight waves in the orange and red portions of the spectrum. Certain1ndicators having the property of indicating a change at approximatelythe point useful in rinse and fixing baths aredi-bromo-thymol-sulfon-phthalein, known as brom thymol blue, which bydaylight changes from yellow to blue and has a range between pH valuesof 6.0 and 7 .6; archil or orchil which changes from pink to lilacbetween 5.6 and 7 .6; phenol-nitro-sulfon-phthalein which changes fromyellow to purple between 6.6

and 8.4 and di-bromo-ortho-cresol-sulfonphthalein, known as brom cresolpurple, which changes from yellow to purple between 5.2 and 6.8. 7

Other indicators for other ranges of acidity which are useful in otherbaths and which also have two markedly different sets of transmissiveproperties to light in the orange and red portions of the spectrum are:tetra-bromophenol-.sulphon-phthalein, known as bromo phenol blue, andtetra chloro-phenol sulfon phthalein, both changing from yellow to bluein the range between 3.0 and 4.6; tetra-bromo-m-cresol-sulfon phthalein,known as brom cresol green, whose range is between pH values of 4.0

PHOTOGRAPHIC BATH.

1925. Serial m. 26,499.

an'd'5.5, and which in daylight changes from yellow to blue; andcochineal, whose range is between pH values of 4.8 and 6.2 and whichchanges from yellow to lilac by daylight, and lacmosol or resorcin blue,changing from red to blue between 4.4 and 5.5.

In processing photographic papers, an acid rinse bath or stop bath isfrequently used between development and fixation. An acid rinse bath isused to neutralize the alkaline developer carried over on the surface ofthe photographic print and thus prevent further progress of development,and to avoid carrying alkaline developer into the fixing bath. This isparticularly undesirable when the latter contains alum as it thenprecipitates a sludge when in an alkaline condition and stains thephotographic paper. A typical formula which complies with therequirements outlined above with reference to a stop bath is thefollowing:

Parts.

ater 1000 Acetic acid (28% pure) 45 Brom thymol blue (0.04) -1 25 Thebrom thymol blue solution should have 3.2 cc. of N/20 sodium hydroxidesolution added per 0.1 gram of indicator.

An indicator for use with an alum fixing bath must be selected so as tochange color just previous to the point where the fixing bath wouldeither tend to stain the emulsion or precipitate a sludge. A typicalformula which complies with these requirements is the following: Sodiumsulphite (desiccated), 12 grams; potassium alum (aluminium and potassiumsulphate), 13 grams citric acid, 6 grams; hypo (sodium thiosulphate),240 grams; brom thymol blue (0.04%), 25 0.0.; water to make 1 liter. Thebrom thymol blue solution should have 3.2 cc. of N/20 sodium hydroxidesolution added per 0.1 grams of indicator. The above amounts would makeonly a small quantity but it is to be understood that they representmerely the proportions ofthe ingredients.

Having thus described our invention, what we claim'as new and desire tosecure y 1. The method of indicating when a bath, useful in photographicprocesses carried out Letters Patent is in light of certain definitelimited wave V proaches such acidity, the original and final 1 colors ofthe bath being ones that are sharply differentiated when viewed by lightof said Wave lengths.

2. The method of indicating when a bath, useful in photographicprocesses carried out. in light of wave lengths in the orange and redportions of the spectrum, approaches a predetermined acidityaffecting'its usefulness that comprises incorporating therein anindicator that causes the color of the 7 bath to change as it approachessuch degree spectrum.

3 .v Abath intended for use after development in a photographic processand containing an indicator whereby the color of the bath Will change inthe Vicinity of pH:6.5, the light transmission characteristics of thebath before and after the color change being markedly different to lightWaves in the orange and red portions of the spectrum.

Signed at Rochester, New York this 2nd day of May, 1925.

JOHN I. CRABTREE. GLENN MATTHEXVS.

